Magnetic tunnel junction (MTJ) element and its fabrication process

ABSTRACT

A magnetic tunnel junction (MTJ) element is provided. The MTJ element includes a reference layer, a tunnel barrier layer disposed over the reference layer, a free layer disposed over the tunnel barrier layer, and a diffusion barrier layer disposed over the free layer. The MU element in accordance with the present disclosure exhibits a low resistance desired for a low-power write operation, and a high TIM coefficient desired for a low bit-error-rate (BER) read operation.

BACKGROUND

Many modern day electronic devices contain electronic memory, such ashard disk drives or random access memory (RAM), Electronic memory may bevolatile memory or non-volatile memory. Non-volatile memory is able toretain its stored data in the absence of power, whereas volatile memoryloses its data memory contents when power is lost. Magnetic tunneljunctions (MTJs) can be used in hard disk drives and/or RAM, and thusare promising candidates for next generation memory solutions. Amagnetic random access memory (MRAM) device is currently explored tofacilitate a static random access memory (SRAM) to own a highnon-volatile storage density. The MRAM device includes an array ofdensely packed MRAM cells. In each MRAM cell, a magnetic tunnel junction(MEI) element is integrated with a transistor to perform write and readoperations.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a schematic view of some embodiments of asemiconductor device including a magnetic tunnel junction (MEI) elementaccording to the present disclosure.

FIG. 2A illustrates a cross-sectional view of some embodiments of an MTJelement according to the present disclosure.

FIG. 2B illustrates a cross-sectional view of some comparativeembodiments of an MTJ element according to the present disclosure.

FIG. 3 illustrates a cross sectional view of some embodiments of amemory device according to the present disclosure.

FIGS. 4-11 illustrate cross-sectional views of some embodiments of amethod of forming a memory device according to the present disclosure.

FIG. 12 illustrates a flowchart representing a method for forming a NMelement according to the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of elements and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” “on” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatially,relative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As used herein, although the terms such as “first,” “second” and “third”describe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another. The termssuch as “first,” “second” and “third” when used herein do not imply asequence or order unless clearly indicated by the context.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in therespective testing measurements. Also, as used herein, the terms“substantially,” “approximately” or “about” generally mean within avalue or range that can be contemplated by people having ordinary skillin the art. Alternatively, the terms “substantially,” “approximately” or“about” mean within an acceptable standard error of the mean whenconsidered by one of ordinary skill in the art. People having ordinaryskill in the art can understand that the acceptable standard error mayvary according to different technologies. Other than in theoperating/working examples, or unless otherwise expressly specified, allof the numerical ranges, amounts, values and percentages such as thosefor quantities of materials, durations of times, temperatures, operatingconditions, ratios of amounts, and the likes thereof disclosed hereinshould be understood as modified in all instances by the terms“substantially,” “approximately” or “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thepresent disclosure and attached claims are approximations that can varyas desired. At the very least, each numerical parameter should beconstrued in light of the number of reported significant digits and byapplying ordinary rounding techniques. Ranges can be expressed herein asbeing from one endpoint to another endpoint or between two endpoints.All ranges disclosed herein are inclusive of the endpoints, unlessspecified otherwise.

A magnetic tunnel junction (MTJ) element includes first and secondferromagnetic films separated by a tunnel barrier layer. One of theferromagnetic films (often referred to as a “reference layer”) has afixed magnetization direction, while the other ferromagnetic film (oftenreferred to as a “free layer”) has a variable magnetization direction.If the magnetization directions of the reference layer and free layerare in a parallel orientation, it is more likely that electrons willtunnel through the tunnel barrier layer, such that the (MTJ) element isin a low-resistance state. Conversely, if the magnetization directionsof the reference layer and free layer are in an anti-parallelorientation, it is less likely that electrons will tunnel through thetunnel barrier layer, such that the (MTJ) element is in ahigh-resistance state. Consequently, the MTJ element can be switchedbetween two states of electrical resistance, a first state with a lowresistance (R_(P): magnetization directions of reference layer and freelayer are parallel) and a second state with a high resistance (R_(AP):magnetization directions of reference layer and free layer areanti-parallel). Because of this binary nature, MTJ elements are used inmemory cells to store digital data, with the low resistance state RPcorresponding to a first data state (e.g., logical “0”), and thehigh-resistance state R_(AP) corresponding to a second data state (e.g.,logical “1”). A performance of the MTJ element is measured by a productof resistance and area (RA), as well as a tunnel magnetoresistance (TMR)coefficient. The TMR coefficient is a ratio of (R_(AP)−R_(P))/R_(P). TheMTJ element is designed to have a low RA mainly for low-power writeperformance, and high TMR coefficient mainly for a broad read windowbetween “0” and “1.”

Typically, an MTJ element is disposed between a bottom electrode and atop electrode, and the reference layer, free layer, and tunnel barrierlayer are manufactured to have a specific crystalline structure andorientation. In some embodiments, the reference layer and the free layermay be made with a body-centered-cubic (bcc) structure with (001)orientation. To attempt to form the MTJ element has this structure andorientation, the tunnel barrier layer having a specific crystallineorientation is applied between the reference layer and the free layer asa template so that the crystalline orientation can be grown in thereference layer and the free layer by a post-annealing process. Forexample, a bcc (001)-textured manganese oxide (MgO) layer can be appliedbetween an amorphous reference layer and an amorphous free layer toinduce crystallization of the reference layer and the free layer with(001) orientation during the annealing process. To form a betterfree-layer crystalline, the MTJ element can be formed with a dual MgOstructure, i.e., by applying an additional MgO layer on a top of thefree layer. However, during the high temperature annealing process, theoxygen atoms in the MTJ element may diffuse out through crystallinegrain boundaries and may migrate towards and be absorbed by elementshaving high oxygen affinity, such as tantalum (Ta), tungsten (W) ormolybdenum (Mo), which adversely affects the RA and TMR coefficient ofthe MTJ element.

The present disclosure therefore provides an MTJ element which includesa diffusion carrier layer formed over the free layer as a cap layer. Insome embodiments, the diffusion carrier layer of the cap layer includesan amorphous, nonmagnetic film of the form X—Z (where X is iron (Fe),cobalt (Co), or the like, and Z is hafnium (Hf), yttrium (V), zirconium(Zr), or the like). In some embodiments, it is found that the resultingMTJ element has a lower RA and a greater magnetoresistance (TMR)coefficient. Consequently, the performance of the MTJ element isimproved. The present disclosure also provides a semiconductor device(e.g., a memory device) including the MTJ element and a method forfabricating the MTJ element and the semiconductor device.

FIG. 1 is a schematic view of a semiconductor device 100 in accordancewith some embodiments of the present disclosure. The semiconductordevice 100 may be a memory device. The semiconductor device 100 includesan MTJ element 102 and an access transistor 104. The MTJ element 102 isdisposed between a bottom electrode 106 and a top electrode 108. Theaccess transistor 104 is coupled to the MTJ element 102 by a first metalwire 110 disposed under a bottom electrode 106. A bit line (BL) iscoupled to one end of the MTJ element 102 through a top electrode 108disposed under a second metal wire 120, and a source line (SL) iscoupled to an opposite end of the MTJ element 102 through the accesstransistor 104. Thus, application of a suitable word line (WL) voltageto a gate electrode of the access transistor 104 couples the MTJ element102 between the BL and the SL. Consequently, by providing suitable biasconditions, the MTJ element 102 can be switched between two states ofelectrical resistance, a first state with a low resistance(magnetization directions of reference layer and free layer areparallel) and a second state with a high resistance (magnetizationdirections of reference layer and free layer are antiparallel), to storedata.

The MTJ element 102 illustrated in FIG. 1 includes a buffer layer 111, aseed layer 112, a hard bias layer 113, an antiparallel coupling (APC)layer 114, a reference layer 115, a tunnel barrier layer 116, a freelayer 117, and a cap layer 118. The reference layer 115 and the freelayer 117 are separated by the tunnel barrier layer 116. The referencelayer 115 has a fixed magnetization, while the free layer 117 has avariable magnetization that can be switched to change between two binarydata states for the MTJ element 102. The hard bias layer 113 is arrangedbelow the reference layer 115 to fix the magnetization of the referencelayer 115 through anti-parallel coupling induced by the antiparallelcoupling (APC) layer 114 arranged between the reference layer 115 andthe hard bias layer 113. In some embodiments, the APC layer 114 isoptional and not a part of the MTJ element 102. The cap layer 118, whichmay also be referred to as a perpendicular magnetic anisotropy (PMA)protection layer in some contexts, is disposed over the free layer 117.

FIG. 2A shows a more detailed example of an MTJ element 102 inaccordance with some embodiments.

The buffer layer 111 is disposed over the bottom electrode 106 and belowthe seed layer 112. In some embodiments, the buffer layer 111 isamorphous and can eliminate unwanted microstructural effects originatedfrom the bottom electrode 106 and facilitate its overlying films todevelop their own desired crystalline structures and orientations. Insome embodiments, the buffer layer 111 is nonmagnetic so that it doesnot interact with the magnetics of its overlying films. In someembodiments, the buffer layer 111 may be or include tantalum nitride(TaNx), in which its nitrogen content can be adjusted to form anamorphous film to facilitate overlying films to grow independently anddevelop their own crystalline structures and orientations. In someembodiments, the buffer layer 111 may be or include an amorphous,nonmagnetic film of the form X—Z (where X is iron (Fe), cobalt (Co), orthe like, and Z is hafnium (Hf), yttrium (Y), zirconium (Zr), or thelike); and in further embodiments, the buffer layer 111 may besubstantially free from Ta or other diffusive species (e.g., ruthenium(Ru)) which may diffuse during high temperature processes (e.g.,annealing). In some embodiments, the buffer layer 111 is a Co—Hf filmwith an Hf content ranging from 18 atomic percentage at. %) to 40 at. %(e.g., 18 at. %, 20 at. %, 25 at. %, 30 at. %, 35 at. % or 40 at. %).The Co—Hf film may have an Co content ranging from 60 at. % to 82 at. %(e.g., 60 at. %, 65 at %, 70 at. %, 75 at. %, 80 at. % or 82 at. %). ACo—Hf film with a Hf content of 18 at % may have 18 percent of thenumber of atoms in the Co—Hf film as Hf, and 82 percent of the number ofatoms in the Co—Hf film as Co; however, in some embodiments, other atomssuch as nitrogen may be doped in the Co—Hf film such that the Co contentmay be lower than 82 at. %. Hf (or Y, or Zr) atoms are much larger thanCo (or Fe) atoms, and thus may distort crystalline lattices to form anamorphous phase. Similarly, nitrogen atoms are much smaller than Co (orFe) atoms, and thus may also distort crystalline lattices to facilitatethe formation of the amorphous phase. Without grain boundaries asdiffusion paths, the buffer layer 111 may act as a diffusion barrierlayer to prevent the hard bias layer 113 and other layers in the MTJelement 102 from diffusion of metallic atoms of the underlying bottomelectrode. In some embodiments, the buffer layer 111 may have a smoothmorphology and a thickness ranging from approximately 1 nm toapproximately 10 nm.

The seed layer 112 is disposed over the buffer layer 111. In someembodiments, the seed layer 112 exhibits a face-centered-cubic (fcc)phase. In some embodiments, the thickness of the seed layer 112 can beadjusted so that the seed layer 112 exhibits a strong <111> crystallinetexture for its overlying films to epitaxially grow, thereby alsodeveloping strong <111> crystalline textures. In some embodiments, theseed layer 112 may include materials such as nickel-chromium (Ni—Cr),nickel-iron-chromium (Ni—Fe—Cr), nickel-iron-nitrogen (Ni—Fe—N) or thelike. In some embodiments, the seed layer 112 includes a nickel-chromium(Ni—Cr) film in a thickness of approximately 6 nm. The Cr content in theseed layer 112 may range from approximately 30 at. % to approximately 50at. % and should be high enough to ensure that the seed layer 112 isnonmagnetic.

The hard bias layer 113 is disposed over the seed layer 112, whichfacilitates the hard bias layer 113 to develop a strong fcc <111>crystalline texture, thereby exhibiting a high perpendicular magneticanisotropy (PMA) and a high coercivity (H_(C)). The hard bias layer 113is a ferromagnetic material having a magnetization direction that isaligned or fixed when applying a high magnetic field in a directionperpendicular to film interfaces. In some embodiments, the hard biaslayer 113 may include a laminated structure of N repeats of alternatingCo and Pt films. In some embodiments, N is an integral number greaterthan one and may be within a range of 3 to 6, but is not limitedthereto. For example, in the embodiments illustrated in FIG. 2A, thehard bias layer 113 may include a multilayer stack 202 composed ofalternating Co and Pt films, and a further Co film 204 disposed over themultilayer stack 202. The multilayer stack 202 is disposed over the seedlayer 112 and may have a structure expressed as [Co/Pt]_(n) where n isan integral number within a range of 3 to 6 and each of the Co and Ptfilms may have a thickness within a range of 0.2 nm to 0.4 nm. Thefurther cobalt film 204 may have a thickness within a range of 0.6 nm to1 nm. The hard bias layer 113 may include any number of layers in anyorder with many suitable materials and thus FIG. 2A is merely anexample.

In some embodiments, the NFU element 102 may include an antiparallelcoupling (APC) layer 114 disposed above the hard bias layer 113 andseparates the hard bias layer 113 from the reference layer 115. The APClayer 114 ensures that the magnetization of the reference layer 115 isopposite to that of the hard bias layer 113 through antiparallelcoupling effect. In some embodiments, the APC layer 114 may be made ofRu and have a thickness of approximately 0.4 nm or within a range ofapproximately 0.3 nm to 0.5 nm; or the APC layer 114 may be made of Irand have a thickness of approximately 0.5 nm or within a range ofapproximately 0.4 nm to 0.6 nm.

The reference layer 115 is disposed over the APC layer 114. Thereference layer 115 is a ferromagnetic layer and has a magnetizationdirection that is “hard-biased” (fixed) by the hard bias layer 113through ferromagnetic coupling and/or antiferromagnetic coupling. Insome embodiments, the magnetization direction of the reference layer 115is opposite to that of the hard bias layer 113. In some embodiments, thereference layer 115 may include two ferromagnetic layers separated by anonmagnetic layer. In some embodiments as illustrated in FIG. 2A, thereference layer 115 includes a first ferromagnetic layer 212 disposedover the APC layer 114, a nonmagnetic layer 214 disposed over the firstferromagnetic layer 212 and a second ferromagnetic layer 216 disposedover the nonmagnetic layer 214. In some embodiments, the reference layer115 may include a cobalt (Co) film 212, a molybdenum (Mo) film 214 andan iron-boron (Fe—B) film 216. In some embodiments, a thickness of theCo film is within a range of approximately 0.6 nm to approximately 1 nm,a thickness of the Mo film is within a range of approximately 0.1 nm toapproximately 0.4 nm, and a thickness of the Fe—B film is within a rangeof approximately 0.6 nm to approximately 1.4 nm.

The Co film of the reference layer 115 has no intrinsic PMA, and isantiparallel coupled with the underlying hard bias layer 113 across theAPC layer 114 to exhibit an extrinsic PMA, A strong antiparallelcoupling may be attained by continuing the epitaxial growth facilitatedby the seed layer 112 and developing the strong fcc [111] crystallinetexture. In addition, the Mo film of the reference layer 115 exhibits abody-centered-cubic (bcc) phase which may terminate the epitaxial growthfor its overlying Fe—B film of the reference layer 115 to grow with anamorphous phase.

The tunnel barrier layer 116, which can manifest as a thin dielectriclayer film, is disposed over the reference layer and separates thereference layer 115 from the free layer 117. In some embodiments, thetunnel barrier layer 116 may include an amorphous film, such as aluminumoxide (AlO_(x)) or titanium oxide (TiO), or a polycrystalline film, suchas manganese oxide (MgO). In embodiments where an MTJ element is used,the tunnel barrier layer 116 is thin enough to allow quantum mechanicaltunneling of current between the reference layer 115 and the free layer117. In some embodiments, the tunnel barrier layer 116 may have athickness ranging from approximately 0.6 nm to approximately 1.2 nm.

In some embodiments, a MgO film is used as the tunnel barrier layer 116.The MgO film of the tunnel barrier layer 116 acts as a PMA promotionlayer which collaborates with the Mo film 214 of the reference layer 115to facilitate the sandwiched Fe—B film 216 of the reference layer 115 toexhibit a strong insitu PMA. In addition, the MgO film of the tunnelbarrier layer 116 exhibits a simple-cubic [001] crystalline textureafter deposition, and may induce a transformation from amorphous topolycrystalline phases also with a [001] crystalline texture in itsunderlying Fe—B film of the reference layer 115 during annealing.Coherent tunneling will occur, thereby increasing the TMR coefficient.

The free layer 117 is disposed over the tunnel barrier layer 116. Thefree layer 117 is capable of changing its magnetization directionbetween one of two magnetization states, which correspond to binary datastates stored in a memory cell. For example, in a first state, the freelayer 117 can have a magnetization direction in which the magnetizationof the free layer 117 is aligned in parallel with the magnetizationdirection of the reference layer 115, thereby providing the MTJ element102 with a relatively low resistance. In a second state, the free layer117 can have a magnetization direction which is aligned antiparallelwith the magnetization direction of the ferromagnetic reference layer115, thereby providing the MTJ element 102 with a relatively highresistance. In some embodiments, the free layer 117 may be depicted as asingle layer. In other embodiments, the free layer 117 may be amultilayer. In some embodiments, the free layer 117 may include twoferromagnetic layers separated by a nonmagnetic layer. In someembodiments, the free layer 117 includes a first ferromagnetic layer 222disposed over the tunnel barrier layer 116, a nonmagnetic layer 224disposed over the first ferromagnetic layer 222 and a secondferromagnetic layer 226 disposed over the nonmagnetic layer 224. In someembodiments, the free layer 117 may include an iron-boron (Fe—B) film222, a manganese (Mg) film 224 and a cobalt-iron-boron Co—Fe—B) film226. In some embodiments, a thickness of the Fe—B film 222 is within arange of approximately 0.8 nm to approximately 1.2 nm, a thickness ofthe Mg film 224 is within a range of approximately 0.3 nm toapproximately 0.5 nm, and a thickness of the Co—Fe—B film 226 is withina range of approximately 0.4 nm to approximately 0.8 nm.

The MgO film of the tunnel barrier layer 116 also acts as a PMApromotion layer which collaborates with the Mg film 224 of the freelayer 117 to facilitate the sandwiched Fe—B film 222 of the free layer117 to exhibit a strong insitu PMA. In addition, the MgO film of thetunnel barrier layer 116 may also induce a transformation from amorphousto polycrystalline phases also with a [001] crystalline texture in itsoverlying Fe—B film 222 of the free layer 117 during annealing. Coherenttunneling will occur, thereby increasing the TMR coefficient.

FIG. 2B illustrates a cross-sectional view of some comparativeembodiments of an MTJ element according to the present disclosure. TheMTJ element of FIG. 2B has a structure similar to that of FIG. 2A exceptthat the configuration of the cap layer 118.

Referring to FIG. 2B, the cap layer 118 is disposed over the free layer117. The cap layer 118 may include a first cap layer 232′ formed of adielectric MgO film disposed over the free layer 117, a second cap layer234′ formed of a ferromagnetic Co—Fe—B film disposed over the first caplayer 232′, a third cap layer 236′ formed of a nonmagnetic Ru filmdisposed over the second cap layer 234′, a fourth cap layer 238′ formedof a nonmagnetic Ta film disposed over the third layer 236′, and a fifthcap layer 239 formed of a nonmagnetic Ru film disposed over the fourthlayer 236′.

The MgO film used as the first cap layer 232′ may have a thicknessranging from approximately 0.4 nm to approximately 1 nm. It also acts asa perpendicular-magnetic-anisotropy (PMA) promotion layer whichcollaborates with the manganese (Mg) film 224 to facilitate thesandwiched Co—Fe—B film 226 to exhibit a strong PMA. As a result, thefree layer 117 exhibits a strong PMA. In some embodiments, to ensure thestrong PMA, the MgO film used as the first cap layer 232′ has an oxygencontent close to or substantially equivalent to that used as the tunnelbarrier layer 116 so as to minimize oxygen chemical potentials betweenthe two MgO films (i.e., 116, 232′), which will lead of substantialoxygen diffusion inside the free layer 117. In some embodiments, oxygendiffusion induced by a large oxygen chemical potential between the twoMgO films may cause an increase in the resistance of the MTJ element 102from 6.3 Ω-cm² to beyond 9.8 Ω-cm², and a decrease in the TMRcoefficient of the MTJ element 102 from 132% to 108% or lower.

The Co—Fe—B film used as the second cap layer 234′ may have a thicknessranging from approximately 0.4 nm to approximately 0.8 nm. The Ru filmused as the third cap layer 236′ may have a thickness ranging fromapproximately 1 nm to approximately 3 nm. The Ta film used as the fourthcap layer 238′ may have a thickness ranging from approximately 1 nm toapproximately 3 nm. The Ru film used as the fifth cap layer 239′ mayhave a thickness ranging from approximately 3 nm to approximately 5 nm.

The Ta film used as the fourth cap layer 238′ has a high affinity tooxygen atoms, and may trap oxygen gases during annealing, therebyprotecting the MTJ element 102 from oxygen penetration from ambientprocessing environments. Without such protection, the oxygen penetrationmay result in a very high RA, and a very low TMR coefficient.

However, in the present disclosure, it is found that the use of the Tafilm 238′ seems to pose some problems. First, it may also trap oxygenatoms in the MgO film used as the first cap layer 232′ through the Rufilm used as the third cap layer 236′ and the Co—Fe—B film used as thesecond cap layer 234′, both of which are not good diffusion barrierlayers at all, thus varying the oxygen chemical potentials between thetwo MgO films (i.e., 116, 232′) and deteriorating TMR properties.Second, it may also even trap oxygen atoms in the MgO film used as thetunnel barrier layer 116 through the free layer 117, which is not a gooddiffusion barrier layer, either, thus also varying the oxygen chemicalpotentials between the two MgO films (i.e., 116, 232′) and deterioratingTMR properties. Third, Ta atoms may penetrate into the underlyinglayers, leading to a loss of magnetic moments of the free layer 117 andthus deteriorating the TMR properties. Due to these concerns, the Ta (Wor Mo also having a high affinity to oxygen atoms) film used as a spacerlayer 224 in the free layer 117 is thus replaced by an Mg film. Fourth,in the junction formation process of the MTJ element 102, Ta atomsetched away may be re-deposited on junctions and react with oxygen gasesto form TaO_(x) at the junctions, leading to concerns on electricalshorting.

Referring back to FIG. 2A, in the embodiments according to the presentdisclosure, the Co—Fe—B film of the second cap layer 234′ (or theCo—Fe—B film of the second cap layer 234′ together with the Ru film ofthe third cap layer 236′) is replaced by a cap layer 118 which acts as adiffusion barrier layer. In some embodiments, the diffusion barrierlayer has an amorphous phase and does not provide Ta and oxygen atomswith grain boundaries as diffusion paths. In some embodiments, thediffusion barrier layer is nonmagnetic so that it will not induceunwanted stray fields to interrupt the operation of the MTJ element 102.In some embodiments, the diffusion barrier layer includes an amorphous,nonmagnetic film of the form X—Z (where X is iron (Fe), cobalt (Co), orthe like, and Z is hafnium (Hf), yttrium (Y), zirconium (Zr), or thelike). In some embodiments, the diffusion barrier layer includes a Co—Hffilm with an Hf content ranging from 18 atomic percentage (at. %) to 40at. % (e.g., 18 at. %, 20 at. %, 25 at. %, 30 at. %, 35 at. % or 40 at.%). The Co—Hf film may have an Co content ranging from 60 at. % to 82at. % (e.g., 60 at. %, 65 at. %, 70 at. %, 75 at. %, 80 at. % or 82 at.%).

It is found that the use of the Co—Hf film in the cap layer 118 canprevent the atoms (such as oxygen) from diffusing out from the element102 and being recaptured by the tantalum (Ta), tungsten (W) ormolybdenum (Mo) film which may affect the performance of the MTJ elementor cause electrical shorting or electrical opening. On the other hand,it can also prevent diffusion of diffusive species, such as tantalum(Ta) or ruthenium (Ru) from the top electrode from or other layers intothe MTJ element 102.

In some embodiments, other atoms such as nitrogen (N) or chromium (Cr)may be doped in or alloyed with the Co—Hf film. In other words, theCo—Hf film may have a nitrogen content ranging from 0 at. % to 30 at. %or a chromium content ranging from 0 at. % to 20 at. % The presence ofnitrogen (N) and/or chromium in the Co—Hf film may further inhibit theformation of unwanted magnetic moments during the annealing process. Insome embodiments, the Co—Hf film may have a thickness ranging from 1 nmto 10 nm, or from 4 nm to 8 nm.

In some embodiments, the cap layer 118 may be depicted as a singlelayer. In other embodiments, the cap layer 118 may be a multilayer. Thecap layer 118 can include any number of layers in any order with manyallowable materials and thicknesses and thus FIG. 2A is merely anexample. In some embodiments, the cap layer 118 may further include amagnesium oxide (MgO) layer 232 disposed below the Co—Hf film (i.e.,Co—Hf layer 234) and over the free layer 117 as illustrated in FIG. 2A.In some embodiments, the MgO layer 232 may have a thickness ranging fromapproximately 0.6 nm to approximately 1 nm. The free layer 117 issandwiched by two magnesium oxide (MgO) films 116 and 232 and protectedby the Co—Hf layer 234. In some embodiments, the cap layer 118 mayinclude an additional cap layer 236 or 238 disposed over the Co—Hf film234 as illustrated in FIG. 2A. Each of the additional cap layers 236 or238 can be independently included in the cap layer 118 or the additionalcap layers 236 and 238 can be both included in the cap layer 118. Theadditional cap layer 236 may include molybdenum (Mo), or tungsten (W),or nickel-chromium (Ni—Cr). The additional cap layer 238 may includetantalum (Ta) or ruthenium (Ru). In some embodiments, each of theadditional cap layers 236 and 238 may have a thickness ranging fromapproximately 1 nm to approximately 10 nm.

In some embodiments as illustrated in FIG. 2A, the cap layer 118 mayinclude a first cap layer 232 formed of a MgO film disposed over thefree layer 117, a second cap layer 234 formed of a Co—Hf film disposedover the first cap layer 232, a third cap layer 236 formed of a Ni—Crfilm disposed over the second cap layer 234 and a fourth cap layer 238formed of a nonmagnetic Ru film disposed over the third cap layer 236.

The Ta film used as the fourth cap layer 238′ in the comparativeembodiments can be eliminated or replaced by a low-resistivity film,such as a Ni—Cr film illustrated in some embodiments according to thepresent disclosure as the third cap layer 236. The Ni—Cr film has a highaffinity to oxygen atoms and a capability of trapping oxygen gasesduring annealing, thereby protecting the MTJ element 102 from oxygenpenetration from ambient processing environments. In some embodiments,the third cap layer 236 may include a nickel-chromium (Ni—Cr) layer in athickness of approximately 6 nm. The Cr content in the third cap layer236 may range from approximately 30 at. % to approximately 50 at. %. Insome embodiments, the Cr content in the third cap layer 236 is highenough to form a nonmagnetic third cap layer 236.

In some embodiments, a Ta-free MTJ structure is “sealed” between twoCo—Hf films, one used in a buffer layer 111 and the other used in a caplayer 118. Unlike Fe—B and Co—Fe—B films which exhibit a “soft”amorphous phase which will be transformed into a polycrystalline phaseduring annealing at a temperature exceeding 300° C., the Co—Hf filmsexhibit a “hard” amorphous phase which will remain as it is duringannealing at a high temperature, such as at 400° C., for a long timeperiod, such as 5 hours.

In some embodiments, with the use of a diffusion barrier layer(specifically, a Co—Hf film) in the cap layer, the present disclosureachieves at least one of the following advantages: a decrease in theresistance of the MTJ element 102 (e.g., from 6.4 Ω-cm² to 5.9 Ω-cm andan increase in the TMR coefficient of the MTJ element 102 (e.g., from124% to 127%). With a low resistance, low voltages can be applied to thetransistor 104 for a low write current to perform low-power writeoperations. With a high TMR coefficient, a separation between codes “0”and “1” will be large enough to minimize a bit-error-rate (BER) whenperforming read operations.

FIG. 3 illustrates a cross sectional view of some embodiments of amemory device 300, which includes MTJ elements 102. The memory deviceincludes a lower conductive wire 304 disposed within a first inter-leveldielectric (ILD) layer 302, a second ILD layer 306 disposed over thefirst ILD layer 302, a lower interconnect via 308 disposed over thelower conductive wire 304, a diffusion barrier layer 310 disposed overthe lower interconnect via 308, a bottom electrode 106 disposed over thediffusion barrier 310 and an insulator layer 314 disposed over thesecond ILI) layer 306. The MTJ elements 102 are disposed between thebottom electrode 106 and a top electrode 108. The bottom electrode 106and the top electrode 108 are conductive, and may include, for example,metals, metal nitrides, or other suitable conductive materials. Forexample, but not limited thereto, the bottom electrode 106 and the topelectrode 108 can include tantalum (Ta), tantalum nitride (TaN),titanium nitride (TiN), tungsten (W), tungsten nitride (WN), platinum(Pt), palladium (Pd), iridium (Ir), nickel-chromium (Ni—Cr), zirconium(Zr), or niobium (Nb). In some embodiments, the lower interconnect via308 and the conductive wire 304 include metal, such as copper ortungsten (W).

The MTJ element 102 includes a buffer layer 111, a seed layer 112 overthe buffer layer 111, a hard bias layer 113 over the seed layer 112, anantiparallel coupling (APC) layer 114 over the hard bias layer 113, areference layer 115 over the APC layer 114, a tunnel barrier layer 116over the reference layer 115, a free layer 117 over the tunnel barrierlayer 116, and a cap layer 118 over the free layer 117. The cap layer118 is disposed between the top electrode 108 and the free layer 117.The cap layer 118 is amorphous and includes a Co—Hf film as discussedabove. The cap layer 118 prevents the atoms (such as oxygen) fromdiffusing out from the MTJ element and being captured by tantalum (Ta),tungsten (W) or molybdenum (Mo) contained in the top electrode 108 orits overlying layers, and prevents diffusion of a diffusive species(such as Ta, W or Mo) from the top electrode 108 to its underlyinglayers. Thus, the RA can be reduced and the TMR coefficient can beincreased.

A spacer 316 covers sidewalls of the MTJ element 102 and the topelectrode 108. In some embodiments, the spacer 316 may also cover a topsurface of the bottom electrode 106 and a top surface of the insulatorlayer 314. In some embodiments, the spacer 316 is formed of siliconnitride. A third ILD layer 318 is disposed over the spacer 316. In someembodiments, the third ILD layer 318 is a tetra-ethyl-ortho-silicate(TEOS) layer. A first dielectric layer 326 is disposed over the thirdILD layer 318. In some embodiments, the first dielectric layer 326 is asilicon carbide (SiC) layer. A second dielectric layer 328 is disposedover the first dielectric layer 326. In some embodiments, the seconddielectric layer 328 is a TEOS layer. A fourth ILD layer 330 is disposedover the second dielectric layer 328. A top electrode via 332 isdisposed over the top electrode 108. A fifth ILD layer 334 is disposedover the fourth ILD layer 330. In some embodiments, the fifth ILI) layer334 is made of a low k dielectric material. A conductive via 336 isdisposed over the top electrode via 332. A conductive wire 338 isdisposed over the conductive via 336. In some embodiments, theconductive wire 338 and the conductive via 336 include metal, such ascopper or aluminum.

FIGS. 4-11 illustrate cross-sectional views 400 to 1100 of someembodiments of a method of forming a memory device according to thepresent disclosure.

As shown in a cross-sectional view 400 of FIG. 4 , an interconnect via308 is formed within a second ILD layer 306. A diffusion barrier layer310′ is formed over the second ILD layer 306. A bottom electrode layer106′ is formed over the diffusion barrier layer 310′. A masking layer401 is formed over the bottom electrode layer 106′. The masking layer401 defines one or more openings 402 a, 402 b and 402 c above a topsurface of the bottom electrode layer 106′.

As shown in a cross-sectional view 500 of FIG. 5 , an etching process isperformed to etch the masking layer 401, the bottom electrode layer106′, and the diffusion barrier layer 310′ and then a bottom electrode106 and a diffusion barrier layer 310 defined by openings 502 a, 502 band 502 c are formed. In some embodiments, an etchant 504 is used.

As shown in a cross-sectional view 600 of FIG. 6 , an insulator layer314′ is formed over the bottom electrode 106. The insulator layer 106fills the openings 502 a, 502 b and 502 c shown in FIG. 5 .

As shown in a cross-sectional view 700 of FIG. 7 , a chemical mechanicalplanarization (CMP) process is performed along line 702. The CMP processremoves a portion of the insulator layer 314′ shown in FIG. 6 and formsan insulator layer 314 shown in FIG. 7 .

As shown in a cross-sectional view 800 of FIG. 8 , a buffer layer 111′,a seed layer 112′, a hard bias layer 113′, an APC layer 114′, areference layer 115′, a tunnel barrier layer 116′ and a free layer 117′are formed over the bottom electrode 106. A cap layer 118′ is formedover the free layer 117′ and a top electrode layer 108′ is formed overthe cap layer 118′. A hard mask layer 810 is formed over the topelectrode layer 108′. A masking layer 801 and photoresist 802 are formedover the hard mask layer 810. The masking layer 801 and photoresist 802are patterned and cover a portion of the hard mask layer 810.

As shown in a cross-sectional view 900 of FIG. 9 , an etching process isperformed using an etchant 902 to remove the portion of the underlyinglayers which are uncovered by the masking layer 801 so as to transferthe pattern of the masking layer 801 to the underlying layers. Theetching process also removes the hard mask layer 810, the masking layer801 and photoresist 802. Then MTJ elements 102 including a buffer layer111, a seed layer 112, a hard bias layer 113, an APC layer 114, areference layer 115, a tunnel barrier layer 116 and a free layer 117 anda cap layer 118 are formed and sandwiched between the bottom electrode106 and the top electrode 108.

As shown in a cross-sectional view 1000 of FIG. 10 , a spacer 316 isformed over the bottom electrode 106 and covers the sidewalls of the topelectrode 108 and the MTJ element 102. A third ILD layer 318 is formedover the spacer 316. A first dielectric layer 326 is formed over thesecond ILD layer 318. A second dielectric layer 328 is formed over thefirst dielectric layer 326.

As shown in a cross-sectional view 1100 of FIG. 11 , a fourth ILD layer330 is formed over the second dielectric layer 328. A top electrode via332 is formed over the top electrode 108. A fifth ILD layer 334 isformed over the fourth ILD layer 330. A conductive via 336 is formedover the top electrode via 332. A conductive wire 338 is formed over theconductive via 336.

The method for forming the MTJ element 102 will be described accordingto one or more embodiments. FIG. 12 is a flowchart representing a method1200 for forming a MTJ element according to aspects of the presentdisclosure. The method 1200 includes a number of operations. It shouldbe noted that the operations of the method 1200 for forming the MTJelement may be rearranged or otherwise modified within the scope of thevarious aspects. It should further be noted that additional operationsmay be provided before, during, and after the method 1200, and that someother operations may only be briefly described herein. Thus, otherimplementations are possible within the scope of the various aspectsdescribed herein.

At operation 1201, a first ferromagnetic layer is formed over a bottomelectrode. In some embodiments, the first ferromagnetic layer isamorphous. In some embodiments, a stack of layers including a bufferlayer, a seed layer, a hard bias layer and an APC layer is formed on thebottom electrode prior to operation 1201. In some embodiments, the firstferromagnetic layer may be formed by using one or more depositionmethods such as, CVD, PVD, atomic layer deposition (AIL)), or the like.

At operation 1202, a tunnel barrier layer is formed over the firstferromagnetic layer. In some embodiments, the tunnel barrier layer has a(001) orientation. In some embodiments, the tunnel barrier layer may beformed by using one or more deposition methods such as, CVD, PVD, atomiclayer deposition (ALD), or the like.

At operation 1203, a second ferromagnetic layer is formed over thetunnel barrier layer. In some embodiments, the second ferromagneticlayer is amorphous. In some embodiments, the second ferromagnetic layermay be formed by using one or more deposition methods such as, CVD, PVD,atomic layer deposition (ALD), or the like.

At operation 1204, a cap layer is formed over the second ferromagneticlayer. The cap layer may include a stack of cap layers. In someembodiments, the cap layer is formed by depositing a first cap layer ofMgO over the second ferromagnetic layer and depositing a second caplayer of CoHf over the first MgO cap layer. The deposition methods mayinclude, but is not limited to, CVD, PVD, atomic layer deposition (ALD),or the like.

At operation 1205, an annealing process is carried out to develop adesired crystalline orientation in the first and second ferromagneticlayers. In some embodiments, the first and second ferromagnetic layershave the same crystalline orientation as the tunnel barrier layer afterthe annealing process. The annealing may be carried out at a hightemperature, such as 350° C. or above. Furthermore, a top electrode maybe formed before or after operation 1205 and an etching process can becarried out after the formation of the tope electrode by a reactive ionetching (RIF) and/or an ion beam etching (IBE), or any suitable method.Thus, an element 102 can be formed and sandwiched by the top electrodeand the bottom electrode.

Accordingly, the present disclosure provides an MTJ element including adiffusion carrier layer as a cap layer, a semiconductor device includingthe element and a method for forming the MTJ element. In someembodiments, the cap layer includes an amorphous, nonmagnetic film of aform X—Z, where X is Fe or Co and Z is Hf, Y, or Zr. The resulting MTJelement has a lower RA and a greater MIR coefficient. Consequently,write and read performance of the MTJ element is improved.

In some embodiments, the present disclosure relates to a magnetic tunneljunction (MU) element. The MTJ element includes a reference layer, atunnel barrier layer disposed over the reference layer, a free layerdisposed over the tunnel barrier layer, and a diffusion barrier layerdisposed over the free layer.

In other embodiments, the present disclosure relates to a semiconductordevice. The semiconductor device includes a magnetic tunnel junction(MTJ) element sandwiched between a bottom electrode and a top electrode.The magnetic tunnel junction (MTJ) element includes a reference layerover the bottom electrode, a tunnel barrier layer disposed over thereference layer, a free layer disposed over the tunnel barrier layer anda cap layer disposed over the free layer. The cap layer includes anamorphous, nonmagnetic film of a form X—Z, where X is Fe or Co and Z isY, or Zr.

In yet other embodiments, the present disclosure relates to a method formanufacturing a magnetic tunnel junction (MTJ) element. The methodincludes the following operations: forming a first ferromagnetic layer;forming a tunnel barrier layer over the first ferromagnetic layer;forming a second ferromagnetic layer over the tunnel barrier layer; andforming a cap layer over the second ferromagnetic layer. The cap layerincludes an amorphous, nonmagnetic film of a form X—Z, where X is Fe orCo and Z is Hf, Y, or Zr.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A magnetic tunnel junction (MTJ) element,comprising: a reference layer; a tunnel barrier layer disposed over thereference layer; a free layer disposed over the tunnel barrier layer;and a diffusion barrier layer disposed over the free layer, wherein thediffusion barrier layer comprises an amorphous and nonmagnetic film of aform X—Z, where X is Fe or Co and Z is Hf, Y, or Zr.
 2. The magnetictunnel junction element of claim 1, wherein the diffusion barrier layercomprises a cobalt-hafnium (Co—Hf) film.
 3. The magnetic tunnel junctionelement of claim 2, wherein the Co—Hf film has a Hf content ranging from18 at % to 40 at %.
 4. The magnetic tunnel junction element of claim 2,wherein the Co—Hf film are doped with nitrogen or alloyed with chromium.5. The magnetic tunnel junction element of claim 1, wherein the MTJelement comprises a cap layer disposed over the free layer and whereinthe cap layer comprises the diffusion barrier layer and an MgO cap layerdisposed between the diffusion barrier layer and the free layer.
 6. Themagnetic tunnel junction element of claim 5, wherein the cap layerfurther comprises a Ni—Cr layer disposed over the diffusion barrierlayer.
 7. The magnetic tunnel junction element of claim 5, wherein thecap layer comprising the diffusion barrier layer and the MgO cap layeris free of tantalum.
 8. The magnetic tunnel junction element of claim 1,wherein the (MTJ) element further comprises an amorphous and nonmagneticlayer disposed below the reference layer.
 9. The magnetic tunneljunction element of claim 8, wherein the amorphous and nonmagnetic layerdisposed below the reference layer comprises a Co—Hf film.
 10. Themagnetic tunnel junction element of claim 1, wherein the free layer is amultilayer.
 11. A semiconductor device, comprising a magnetic tunneljunction element sandwiched between a bottom electrode and a topelectrode, wherein the magnetic tunnel junction element comprises: areference layer over the bottom electrode; a tunnel barrier layerdisposed over the reference layer; a free layer disposed over the tunnelbarrier layer; and a cap layer disposed over the free layer; wherein thecap layer comprises an amorphous, nonmagnetic film of a form X—Z, whereX is Fe or Co and Z is Hf, Y, or Zr.
 12. The semiconductor device ofclaim 11, wherein the amorphous, nonmagnetic film of the cap layer is aCo—Hf film.
 13. The semiconductor device of claim 11, wherein themagnetic tunnel junction element further comprises an amorphous,nonmagnetic film of a form X—Z, where X is Fe or Co and Z is Hf, Y, orZr, disposed below the reference layer.
 14. The semiconductor device ofclaim 11, wherein the reference layer comprises a cobalt (Co) layer, amolybdenum (Mo) layer and an iron-boron (Fe—B) layer.
 15. Thesemiconductor device of claim 11, wherein the tunnel barrier layercomprises aluminum oxide, titanium oxide, or a manganese oxide.
 16. Thesemiconductor device of claim 11, wherein the free layer comprises aniron boron (FeB) layer, a manganese (Mg) layer and a cobalt-iron-boron(Co—Fe—B) layer.
 17. The semiconductor device of claim 11, wherein thesemiconductor device is a magneto-resistive random-access memory (MRAM)device.
 18. A memory device, comprising: a magnetic tunnel junctionelement sandwiched between a bottom electrode and a top electrode,wherein the magnetic tunnel junction element comprises: a referencelayer over the bottom electrode; a tunnel barrier layer disposed overthe reference layer; a free layer disposed over the tunnel barrierlayer; and an amorphous diffusion barrier layer disposed over the freelayer, wherein the amorphous diffusion barrier layer comprises anonmagnetic film of a form X—Z, where X is Fe or Co and Z is Hf, Y, orZr; and a spacer layer over sidewalls of the magnetic tunnel junctionelement and the top electrode.
 19. The memory device of claim 18,wherein the diffusion barrier layer includes a Co—Hf film with an Hfcontent ranging from 18 at. % to 40 at. %.
 20. The memory device ofclaim 18, wherein the free layer is a multilayer.